Fluxes for sintering lithium ferrites



United States Patent 3,300,411 FLUXES FOR SINTERING LITHIUM FERRITES Donald G. Wickham, Santa Monica, and Ho Bin Im, Los

Angeles, Calif., assignors to Ampex Corporation, Culver City, Calif., a corporation of California No Drawing. Filed Feb. 21, 1964, Ser. No. 346,399 5 Claims. (Cl, 252-625) This'invention relates to improved lithium ferrites. More particularly, the invention relates to a method for improving lithium ferrites utilizing fluxes to promote sintering.

Ferromagnetic materials have found particular application in computer memory construction. More specifically, ferromagnetic materials of the ferrite type are widely used in the manufacture of magnetic memory cores. Materials such as ceramic lithium ferrite have unusually good properties for such applications. Lithium ferrite has what may be considered a square hysteresis loop characteristic in that it can be switched between opposite stable magnetic states by a current pulse of suflicient amplitude, when used as a core in a memory device. The required switching amplitude of the pulse is dependent upon the coercivity of the material. Lithium ferrite has a small temperature coefficient of coercivity as one of its more important properties. This enables the device in which the core is utilized to be subjected to wide ranges of temperatures without the coercivity being greatly affected. Despite the low temperature coefficient of coercivity, lithium ferrite has been limited in use previously because its value of coercivity is relatively high as compared to other commonly used ferrite materials. Temperatures in excess of 2100 F. would be required to fire the ceramic lithium ferrite in order to produce a sufliciently dense material which would have a coercivity comparable to other ferrites. Below 2100 F. the grain growth of the ceramic does not proceed to make the material as dense as required. At 2100 F. or above however, rapid evaporation of the lithium transpires which obviously changes the composition. With the resultant loss of lithium, the square loop characteristic of the material is greatly decreased. Additionally, there is a lack of uniformity among the cores produced, from one batch to another. Core specifications become difficult .to meet and often cannot be reached at such high temperatures.

It is an object of this invention to provide a method of making lithium ferrite and the composition resulting therefrom with a low value of coercivity.

Another object of the invention is to provide a method of making lithium ferrite and the composition resulting therefrom without utilizing a firing temperature above 2000 F.

Still another object of the invention is to provide a method for making lithium ferrite having a low value of coercivity without greatly affecting the temperature coeflicient of coercivity of the material.

These and other objects of the invention are accomplished by the utilization of lithium vanadates as fluxes to lower the firing temperatures of lithium ferrite and causes densification thereof. lithium vanadates are intimately mixed with the lithium composition prior to hiring, whereby a resultant final composition has the firing chemical formula wherein M represents at least one divalent cation usually found in spinel ferrites and is selected from the class consisting of Mg, Mn, Fe, Co, Ni, Cu, and Zn and x does not exceed .05. Preferably, x is within the range of .005 to In the method of this invention, the p was removed from the mill and dried.

. mlately one and one-half hours.

. .again in a ball .mill andthe resultant slurry was dried and .025. The metal cation M is provided so that a correct balance of the valences is present in the resultant mate-rial. .It is necessitated by the presence of the vanadium which has the 5+ valence. The lithium vanadates or other vanadate hydrates can be utilized as fluxes in this invention. Lithium vanadates which can be used include, but are not limited to, lithium orthovanadate, Li VO metavanadate LiVO pyrovanadate Li V O oxyorthovanadate Li O-2Li VO -H O. The orthovan adate hydnates include Li VO -8H O and Li VO -H O.

Prior to this invention, vanadium pentoxide V 0 had been used as a flux to improve the properties of Ni-Zn ferrites and magnesium ferrites. In order to act as a flux without deleteriously affecting the resultant material, vanladium pentoxide was required in very small quantities. As a result, it was difiicult to thoroughly mix with the bulk of material and could not adequately perform its function as a flux. The lithium vanadates of this invention have many advantages over the vanadium pentoxide previously utilized. They are generally of lower density than the vanadium pentoxide and therefore occupy greater volume per the vanadium atom and are more easily mixed. Additionally, some of the lithium vanadates such as Li VO Ll4VOzO7 have higher melting points than vana-. dium pentoxide and fluxing action is more easily controlled. It is believed that the invention will be better understood from the following detailed explanation and examples. EXAMPLE I A composition containing vanadium in accordance with the chemical formula Li+ V M Fe O as previously set forth was prepared. The value of x was chosen to equal .01. The constituents used to make the material were carefully weighed out and then placed in a ball mill for grinding. The compounds utilized included 0.235 mole (17.36 g.) of lithium carbonate, Li CO 0.01 mole (1.358 g.) of lithium orthovanladate, Li VO 0.02 mole (1.49 g.) of nickel. oxide and 1.235 moles (197.2 g.) of ferric oxide, Fe O A grinding medium was chosen in which all the foregoing constituents were insoluble. Isopropyl alcohol was utilized for this purpose. The mixture was then milled for eight hours.

It was then sieved and reacted at a temperature of 1400 F. for approxi- The material was ground sieved. It was then prepared for pressing by the addition of a binder and lubricant. A polyvinyl alcohol, known as Elvanol land made by Du Pont Company, was added in the amount of 2 /2 weight percent to serve as the binder. The lubricant consisted of /2 weight percent stearicacid.

Cores of desired size were pressed from the material inthe fluxes, may be any that will form an oxide of the metal upon heating. Thus, for example, oxa'lates, nitrates, hydroxides, and carbonates of lithium, and the divalent 1 metal cation are contemplated.

In the preparation of the cores in accordance with the above example, the amount and type of binders and lubricants utilized are well known in the art. Any suitable The resultant slurry compounds will serve. The weight percent of the materials will increase with sibe of the cores being made. Some compounds can serve the dual purpose of lubricant and binder, such as Carbowax.

EXAMPLE II The above example was repeated utilizing varying values of x in the above formula of the invention so that differing amounts of the lithium vanadate fiux were present. Results indicating the performance of the compounds of this invention as compared to a compound wherein no flux is utilized is indicated in Table I below.

In the Table I, H represents the coercivity; I /I is the maximum disturb ratio wherein l isthe maximum sults. Equally successful results are obtained when utilizing any of the other lithium vanadates or orthovanadate hydrates previously described as fluxes in accordance with the procedure of Example I.

EXAMPLE III To further indicate the outstanding properties of the invention, additional smaller cores were prepared and tested. The cores had an outside diameter of 0.030 inch and inside diameter 01501020 inch and were 0.007 inch high. The changes of the properties of the cores with temperatures under typical operating conditions are listed in the following Table II. The composition of the w Was o.s 0.o1 o.o2 62.47 4- Firing temperature 1,900 1*.

partial \vrlte/Ilull v 363 ma.

11: 0.2 ,u see.

amplitude of a disturb pulse; I is the amplitude of the full-write pulse; ,uV is the undisturbed one output voltage; dV is the disturbed zero response voltage; t is the switching time in microseconds and the temperature range in C. represents the useful temperature range for the core. The core utilized had an outside diameter of 0.050 inch and an inside diameter of 0.030 inch and was 0.015 inch in height. The rise time t of the current pulses were 0.5 microsecond. The temperautre was C.

It should be particularly noted that over the wide temperature range of to ,+125 C. the disturbed output voltage, dV about doubled indicating the extreme value of the compound of the invention over such temperature ranges. This is a most critical factor to observe in performance of the cores. The remaining properties of the cores, as can be seen from the tables, remained essentially constant over the wide temperature range, further indicating their particular usefulness.

There has accordingly been described herein a novel,

Table I PROPERTIES OF LiMV Mg FQ -MOA MEMORY CORES (25 C.)

I 0 :c 0.01 2 0.014 :c 0.02

Firing temp. F.) 2,000 1,950 1, 950 1,900 11,, (amp-turns) n 1. 7 0. 95 0.80 0.70 I 1. 1 2 0. 640 0.520 0. 440

b w 1. 70:0. 66 0. 950 0. 68 0. 80020. 65 0. 700 0. 63 N (mv.) 80 80 70 11V, 7 4 5 5. 5 t. (,tsec.) 1. 05 1.05 1. 05 1.05 Temp. range 0.). -25 to 125 -50 to 125 50 to 100 -50 to 75 A most important factor, as seen from the above table, is that without the presence of the vanadate flux of the invention, the coercivity of the lithium ferrite was extremely high at a level of 1.7, indicating an excessive amount of current would be necessary to operate the core. This value was substantially reduced, due to the utilization of the flux, to a value of less than one-halfwhere x equals 0.2. To attempt to lower the coercivity of the core in the column, where x equals 0, in other words where no flux is present, the firing temperature would of necessity have to be raised above 2100" F. As previously indicated, it is quite detrimental, causing vaporization of lithium, producing unpredictable cores having poor properties.

In addition, it is noted from the above table that with the presence of the lithium vanadate fluxes in accordance.

with this invention, the disturbed zero" response voltage,

dV was substantially reduced, indicating better per formance for the compounds of the invention. 'It is noted that nickel was the metal atom utilized in the compounds. As previously indicated, any of the other metal atoms found in pure spinel ferrites would producesuccessful re- 75 useful method and means for making improved lithium ferrite material.

What is claimed is:

1. In the method of making lithium ferrites which includes the steps of milling compounds of iron and lithium capable of forming an oxide upon heating, mixing said compounds with binders and lubricants, pressing said mixed compounds into a desired shape, and heating said pressed compounds to a temperature sufilcient to produce said lithium ferrites, the improvement comprising mixing with conventional lithium ferrite starting compounds, a flux selected from the class consisting of lithium v anadates and lithium orthovanadate hydrates.

2. The improvement of claim 1 wherein the lithium vanadate is lithium orthovanadate.

3. In the method of making lithium ferrites which includes the steps of milling compounds of iron and lithium capable of forming an oxide upon heating, mixing said compounds with binders and lubricants, pressing said mixed compounds into a desired shape, and heating said pressed compounds to a temperature sufiicient to produce 5 6 said lithium ferrite, the improvement which comprises References Cited by the Examiner milling with said starting compounds a flux selected from UNITED STATES PATENTS the class consisting of lithium vanadates and lithium orthovanad-ate hydrates, whereby the temperature required giekarski t f 'dl'th" f 't '1 th 2000-F. 1 Town o ormsal 1 mm emels 688 an 5 3,208,948 9/1965 Blasse 25262.5

4- The method of claim 3 additionally comprising milling with the starting materials a divalent metal cation com- I pound capable of forming an Oxide upon heating HELEN M. MCCARTHY, Acting Puma; y Examiner.

5. The method of claim 3 wherein said flux is lithium TOBIAS E. LEVOW, Examiner.

Orthovanadate- 10 R. D. EDMONDS, Assistant Examiner. 

1. IN THE METHOD OF MAKING LITHIUM FERRITES WHICH INCLUDES THE STEPS OF MILLING COMPOUNDS OF IRON AND LITHIUM CAPABLE OF FORMING AN OXIDE UPON HEATING, MIXING SAID COMPOUNDS WITH BINDERS AND LUBRICANTS, PRESSING SAID MIXED COMPOUNDS INTO A DESIRED SHAPE, AND HEATING SAID SAID LITHIUM FERRITES, THE IMPROVEMENT COMPRISING MIXING WITH CONVENTIONAL LITHIUM FERRITE STARTING COMPOUNDS, A FLUX SELECTED FROM THE CLASS CONSISTING OF LITHIUM VANADATES AND LITHIUM ORTHOVANADATE HYDRATES. 